This work aims to elucidate molecular events during early amphibian embryogenesis. To this end molecular markers specific for early differentiation events are being isolated and studied. Recent work has focused on two systems: (1) Reconstituted expression of keratin genes injected into the embryo. (2) Embryonic induction. Keratin genes have been shown previously to be expressed in the ectoderm only. Constructs of keratin genes have been injected into fertilized Xenopus eggs, resulting in temporally and spatially regulated expression of these introduced genes. Induction of mesoderm has been studied with the aid of culture fluid of the Xenopus cell line named XTC. A factor or factors in this medium induces morphological and biochemical differentiation towards mesodermal derivatives in explants (animal caps) that otherwise would become ectoderm. A relationship between the inducing principle in XTC medium and transforming growth factor beta-2 (TGF-beta 2) been established by showing that mammalian TGF- beta 2 induces mesoderm in animal cap explants; this activity is blocked by antibody against TGF-beta 2. Purification of the active factor or factors from XTC medium is underway. Further, several cDNA clones have been isolated that are rapidly induced in animal caps by XTC factor; some of these clones also respond to fibroblast factor (FGF) which has been implicated in certain aspects of mesoderm induction. Study of these clones may help analyze signal responses in the embryo.